1. Field of the Invention
The present invention relates to a wavelength division multiplexed (WDM) system, and more particularly to a WDM system including a mode-locked light source.
2. Description of the Related Art
A WDM system assigns a specific wavelength to each subscriber to facilitate an ultra high-speed wideband communication service. The WDM scheme is advantageous in that it can easily increase the channel capacity and can ensure a high level of security for a user as it assigns a specific wavelength to each subscriber. In addition, a separate wavelength is assigned to a new subscriber, so that new subscribers can be easily accommodated in a communication network.
Recently, a mode-locked light source for generating mode-locked optical signals using incoherent light has been used in the aforementioned WDM communication system. The mode-locked light source includes a Fabry-Perot laser for generating mode-locked optical signals, a wideband light source for generating a light over a wide wavelength range for inducing the mode-lock of the Fabry-Perot laser, etc. The wideband light source may use an Erbium doped optical fiber, a semiconductor optical amplifier, etc. The light, which may be spontaneous emission light, is used for mode-locking a corresponding Fabry-Perot laser after being divided according to each wavelength.
A WDM communication system using the mode-locked light source as described above includes a Passive Optical Network (PON). The PON provides optical signals to multiple subscribers through a single line. The PON includes a central office for providing mode-locked downward optical signals, a plurality of subscriber sides for receiving corresponding downward optical signals and generating mode-locked upward optical signals, and a remote node for relaying the subscriber ends to the central office. The central office is connected to the remote node through a single optical fiber.
The remote node includes a multiplexer/demultiplexer so as to multiplex the upward optical signals received from the subscriber side and output the multiplexed upward optical signal to the central office. The central office demultiplexes multiplexed downward optical signals and outputs the demultiplexed downward optical signals to the subscriber sides.
The central office includes a multiplexer/demultiplexer, downward wideband light sources for generating the mode-locked downward optical signals, photodetectors for detecting corresponding demultiplexed upward optical signals, a controller for monitoring and controlling wavelength change of upward optical signals and downward optical signals, monitoring means for monitoring the wavelength change of the upward optical signals and the downward optical signals and normality or abnormality of a network, a optical module for generating lights over a wide wavelength range, etc.
The optical module further includes a first light source for generating a downward light over a wide wavelength range for mode-locking the downward wideband light sources, a second light source for generating an upward light for mode-locking each of the subscriber sides, and a first optical coupler for inputting/outputting multiplexed upward and downward optical signals through the optical fiber and connecting the first light source with the a second light source through the optical fiber.
The monitoring means monitor whether the line of the PON functions properly. Further, the monitoring means monitor downward or upward optical signals and transmit the wavelength change in the downward optical signals or the upward optical signals to the controller. The controller compares the wavelengths of the downward optical signals or the upward optical signals, which are received from the monitoring means, with predetermined input values. Then, the controller controls the PON according to the wavelength change obtained from the comparison result.
However, the conventional WDM PON including the mode-locked light source inputs/outputs the light over the wide wavelength range and the mode-locked optical signals through the optical couplers which tend to cause unnecessary loss in the intensity of light.